A survey of 39,129 people in the US found that 42% of women with regular menstrual cycles had heavier than normal bleeding after being vaccinated against the coronavirus.
However, its authors - Katharine Lee, professor in the Department of Anthropology at Tulane University, New Orleans; and Eleanor J. Junkins, professor in the Department of Psychology at the University of Illinois at Urbana-Champaign, acknowledged that although "40% of this sample has this experience, it does not mean that this is the rate in the world."
The study , published in the scientific journal Science Advances , was carried out from a survey launched on April 7, 2021 for 12 weeks, initially on Twitter, but spread quickly across multiple social media and media platforms. .
The authors began the investigation after sharing in early 2021 that they had unexpected menstrual bleeding after inoculation.
Many participants found out about the survey after conducting an internet search to investigate their own experiences and find social networks or news coverage of this project, according to the Télam agency.
The people surveyed were between 18 and 80 years old, all participants were fully vaccinated with Pfizer, Moderna, AstraZeneca, Johnson & Johnson, among other vaccines (at least 14 days after one or two required doses, as this was before the reinforcements) and had not contracted Covid-19 (diagnosed or suspected).
To investigate emerging changes in menstrual bleeding patterns, they sampled 39,129 people - 90% identified only as women and 9% as gender diverse - who menstruate now and previously.
The study showed that 42% of people with regular menstrual cycles bled more than normal, while 44% reported no changes after being vaccinated, the authors detailed.
Also people surveyed who do not normally menstruate reported breakthrough bleeding after vaccination, including: 71% of people taking long-acting reversible contraceptives, 39% of people taking gender-affirming hormones, and 66% of people taking gender-affirming hormones. of postmenopausal people.
In this regard, they found that increased or intermenstrual bleeding was significantly associated with age, systemic side effects of the vaccine (fever or fatigue), history of pregnancy or childbirth, and ethnicity.
Their main conclusions were that "heavier menstrual flow was more likely for those respondents who were non-white, were Hispanic/Latina, were older, had a diagnosed reproductive condition, used hormonal contraceptives, had been pregnant in the past (already whether they had not given birth), or experienced fever or fatigue after vaccination.
In turn, the researchers explained that vaccines "work by mobilizing the immune system to protect it from diseases if exposure occurs" and this immune activation "is important, although it can also produce a cascade of other localized inflammatory responses (e.g. , pain at the injection site) or systemic (e.g., fatigue and/or fever)", in statements cited by the news agency.
In that sense, they pointed out that studies evaluating the direct effect of vaccination on the menstrual cycle are few and far between, and gave as an example one from 1913 that identified that the typhoid vaccine was associated with menstrual irregularities, or studies of hepatitis B and human papillomavirus have also indicated that menstruation could be altered.
Regarding the study design, they reported that it was "observational and retrospective of vaccinated people" and the survey was "exploratory and mixed methods" intended to capture a wide range of responses with the goal of "providing a description of trends for both both doctors and the public.
Finally, in dialogue with the Spanish newspaper El País, researcher Lee defended the safety of vaccines and reinforced the idea expressed in the study that the associations described "are not causal", but "they provide evidence to better study these trends." .
